EQUINE INFLUENZA VACCINES AND THE
IMPLICATIONS OF THEIR USE IN NEW ZEALAND
The recent news that Australia has begun the controlled use
of equine influenza (EI) vaccine in infected areas has resulted
in increasing calls from a number of quarters for vaccination
of horses in New Zealand. The use and timing of vaccination
in an EI outbreak is always a controversial issue. A decision to
vaccinate, especially in situations where the disease is not
present, is not a simple matter and many factors require
consideration. The impacts and consequences of vaccination
will also affect different sectors of the horse industry in different
ways, and it is important that all views from potentially affected
parties are considered before any decisions are made. To be
able to make an informed decision it is important that all the
facts around the vaccine be considered.
Vaccine types
There are three types of EI vaccines available:
Killed or inactivated vaccines – the vaccine contains virus that is killed (usually by chemicals). These vaccines
frequently include a number of strains of the EI virus. The main advantage of these vaccines is their safety. They are
administered by injection and usually require two injections four to six weeks apart. Vaccinated animals usually only
become fully immune 7–14 days after the second dose. For ongoing protection from disease a booster vaccination
is given at six months, and thereafter yearly or more frequently, depending on the likely exposure of the horse to EI.
Live modified vaccines – these vaccines have been made relatively safe through a process whereby the virulence of
the virus is weakened. Because a live virus is used they do present the risk of viral spread. Vaccine is usually
administered via the nose of the animal. These vaccines must not be used in pregnant mares. A single dose is followed
by boosters at six monthly intervals. There is evidence of early protection (as early as seven days) following vaccination,
though full immunity is claimed to occur later than this.
Modified vaccines – another form of “live vaccine” they include genetically modified virus material and are treated
as a separate type of vaccine. The virus, because it has been modified, is not able to spread from horse to horse. The
vaccine is given by injection with two initial doses given four to six weeks apart, followed by boosters at six month
intervals. Immunity has been documented at 14 days after administration of the first dose.
Which vaccine to use?
Like the human flu virus, the EI virus is constantly changing and this is the reason why horses can catch influenza
more than once, or become infected even if they are vaccinated.
The different types of vaccine are named after the place where the disease was confirmed – you will see reference
to names like Newmarket, Florida, Prague etc. Although vaccination may prevent disease, EI vaccines neither fully
prevent infection nor transmission of the virus.
It is important to use the correct vaccine for the type of influenza virus you want to protect against (i.e. that the
vaccine contains the correct virus types). Vaccine manufacturers are constantly updating their vaccines in response
to new subtypes. When there is a new outbreak it is important to know the virus type you are dealing with, to be able
to select the most appropriate vaccine. Most influenza vaccines have at least two subtypes and generally include
American and European strains. Correctly vaccinated horses shed less virus for shorter periods and show fewer or no
detectable clinical signs than horses that have not been vaccinated.
Immunity is short lived and costly
The immunity in horses after vaccination is relatively short lived and therefore revaccination is important. It is generally
accepted that for vaccination to have much impact on EI it is necessary to have a strict vaccination programme that
includes about 70% of the horse population. If we were to apply this to New Zealand with approximately 120,000
horses, we would need to vaccinate 84,000 horses. At a cost of approximately $375 + GST for an initial three dose
course of killed vaccine per horse the first year cost would be about $30 million.
If a vaccination programme is not maintained the population may become increasingly susceptible to EI again. This
is one of the reasons why in countries where vaccination is practised there are still outbreaks of the diseases.
Vaccinated horses can spread the disease
Vaccinated horses may still become infected but will show few or no clinical signs because they have some protection.
Horses that have had EI or that have been exposed to the virus (even if vaccinated) can carry and shed the virus for
up to 14 days and infect other animals. Therefore the disease can be spread as horses that are not obviously ill are
moved from place to place. This is often how the disease enters a country – in horses with immunity that are carrying
the virus. Vaccination also makes confirming the presence of EI more difficult and more expensive.
Why use vaccine in an outbreak?
If used correctly in an outbreak vaccines can slow the spread of the disease and reduce or minimise the effects in
horses. However, it is important that the use of vaccines is strictly controlled, so that it is known which horses have
been vaccinated and when they were vaccinated, to ensure that vaccinated horses do not unwittingly spread the
disease further.
Registered vaccines
MAF Biosecurity New Zealand (MAFBNZ) currently has two vaccines registered (both inactivated vaccines) for
emergency use in the event of an EI outbreak in New Zealand. The vaccines are registered to enable MAFBNZ to
quickly source stocks from overseas. No stocks of vaccine are actually held in New Zealand. In an outbreak the plan
is to be able to vaccinate unaffected horses away from an infected area to create a “buffer zone” while movement
controls are put in place and eradication attempted. However, as inactivated vaccines may take quite a long time to
produce immunity in horses other vaccines are also being considered.
Vaccination in non-emergency situations
Since Australia decided to vaccinate against EI the issue of why New Zealand should or should not vaccinate has
caused a lot of debate.
There are several reasons why the vaccine is strictly controlled and why using it preventatively in the absence of
disease needs to be very carefully considered.
Possible uses for the vaccine in the absence of EI in New Zealand could include:
targeted vaccination of high risk populations within New Zealand e.g. horses that travel frequently out of New
Zealand, or
pre-emptive vaccination of the country’s horse population to reduce the impact of disease and to reduce the
likelihood of spread should it enter New Zealand.
If pre-emptive vaccination is used in New Zealand at least 70% of the horse population needs to be immune, so that
the disease does not spread. If lower percentages of horses are vaccinated then this may not prevent outbreaks from
occurring. And even with high vaccination rates there is no guarantee of protection, as other strains may emerge that
horses are not immune to and still result in outbreaks.
Implications of using vaccination before the disease occurs
Impact on EI free status for international trade
One of MAFBNZ’s main concerns around vaccination on a large scale in the absence of disease (i.e. non-emergency
use) is the potential impact on our EI free status.
If a significant number of horses were vaccinated then MAFBNZ could no longer rely upon detecting clinical signs
in infected animals, as we currently do, to ensure that New Zealand retains its EI free status. This could affect exports
to other nations. New Zealand would still claim freedom but this would be treated with less credence by trading
partners who are under no obligation to accept our claims of freedom at face value.
The International Organisation for Animal Health (OIE) has specific guidelines for EI free countries where vaccination
is used. To comply with their requirements New Zealand’s equine industry would need to undertake an extensive and
expensive surveillance programme, to ensure that our claims of EI freedom are accepted by other countries. This also
means that to retain EI free status, New Zealand would need to identify horses that have been vaccinated, which is
why all horses that are vaccinated in Australia have to be micro-chipped. If maintaining our EI free status is not
considered important then these measures and associated costs may be avoided.
“Silent spread”
Vaccination may hide signs of infection in horses but still allow horses to spread the virus. This will result in so-called
“silent spread” of the disease. This would allow the disease to spread much more rapidly before it could be detected.
By the time it shows up in a non-vaccinated horse it could potentially be very widespread.
Need for repeated, regular, vaccination
Horses need to be vaccinated at regular intervals to ensure that a solid immunity is built up and maintained. If
vaccination is allowed to lapse (and this is a real risk where people don’t see evidence of disease after a period of
time and then decide not to revaccinate) it results in a horse population that is again at risk.